1. Field of the Invention
The present invention relates to a glass composition used for the fabrication of an optical device, and more particularly to a glass composition for an optical fiber used in light amplification. It also relates to an apparatus for optical communications using the glass composition.
2. Description of the Related Art
Silica glass fiber doped with an active material, i.e., erbium (Er) is currently used as the optical fiber in a light source such as a laser oscillator of a single wavelength used for optical communications, a superluminescent source of radiation, or an amplifier of radiation. The silica glass fiber doped with Er is used for amplifying a 1.5 .mu.m wavelength signal.
An optical fiber used for amplifying a 1.31 .mu.m wavelength signal which is the Zero dispersion bandwidth of silica glass has not, however, been successfully developed. Two rare earth elements are potentially useful as the active material in an optical fiber used for optical amplification of the 1.3 .mu.m wavelength signal: neodymium (Nd) and praseodymium (Pr) which would be doped on a host glass. (By way of definition, a glass which is not doped with an active material will be referred to as host glass. The term glass composition will mean a host glass doped an the active material.) The rare earth element is doped in an ionic state, such as Nd.sup.3+ or Pr.sup.3+ ions, on a host glass such as silica glass.
However, in the case of including the Nd.sup.3+ ion, the center of the luminescence waveband generated by the transition of the Nd.sup.3+ ion from .sup.4 F.sub.3/2 level to .sup.4 F.sub.13/2 level is about 1.35 .mu.m, which is considerably spaced from the Zero dispersion bandwidth. Also, the luminescence at 1.35 .mu.m is weaker than those of other emissions generated from the .sup.4 F.sub.3/2 level, for example, 0.89 .mu.m and 1.064 .mu.m. Furthermore, the gain of light of wavelength shorter than 1.36 .mu.m is remarkably reduced by an excited state absorption in the .sup.4 F.sub.3/2 level.
In the case of adding the Pr.sup.3+ ion, light generated by the transition between a higher energy level .sup.1 G.sub.4 and a lower energy level .sup.3 H.sub.5 is used as a signal. Here, the probability of this transition is much larger than the probability of the transition from the .sup.1 G.sub.4 to other energy levels than the .sup.3 H.sub.5. However, the difference of an energy gap between the .sup.1 G.sub.4 level and the .sup.3 F.sub.4 energy level which is right below the .sup.1 G.sub.4 level is about 3,000 cm.sup.-1. Therefore, in the case of using oxide glass having a large lattice vibration energy (&gt;800 cm.sup.-1) as a host material the probability theat the energy of an electron excited to the .sup.1 G.sub.4 level in Pr.sup.3+ ion is consumed by nonradiative transfer increases with the relaxation of lattice vibration energy. As a result, the light amplification efficiency becomes low. Therefore, it is necessary to use glass having a low lattice vibration energy as the material of the host.
One such system for a host glass having a low lattice vibration energy is disclosed in U.S. Pat. No. 5,379,149, to Snitzer et al., entitled Glass Composition Having Low Energy Phonon Spectra And Light Sources Fabricated Therefrom. The patent discusses a host glass having a composition in which excess S is added in a ratio higher than S ratio on a composition line which connects GeS.sub.2 and Ga.sub.2 S.sub.3 in a ternary system phase diagram of germanium (Ge), gallium (Ga) and sulfur (S). This is referred to as a sulfur-rich glass. In particular, the patent discusses a composition of Ga, S and Ge with the above limitation and further limited to values of S less than 75 mol % and Ga less than 10 mol %. The composition with 25 mol % Ge, 5 mol % Ga and 70 mol % S, which may also be represented Ge.sub.25 Ga.sub.5 S.sub.70, was about at the center of the discussed composition region on the ternary phase diagram.
The host glass of the Ge.sub.25 Ga.sub.5 S.sub.70 composition has a higher solid solubility of the Pr.sup.3+ ion than the host glass of conventional Ge--S, As--S, or Ge--As(P, Sb)--S based compositions. However, when the Pr.sup.3+ ions is added at high concentration, agglomeration of the Pr.sup.3+ ions occurs. The agglomeration of the Pr.sup.3+ ions causes the energy transfer rate between the Pr.sup.3+ ions to greatly increase. Therefore, the luminescence by the .sup.1 G.sub.4 level life is reduced and the light amplification efficiency is lowered.
Based on our observations of the art, we have found that what is needed is a glass composition for light amplification which has a low lattice vibration energy to permit efficient luminescence from the rare earth ion, but which prevents agglomeration of the rare earth ions thereby further enhancing the luminescence.